Copending application Ser. No. 08/346,996 describes a Vertical ElectroEtch Tool (hereafter referred to as VEET) used in the electrochemical fabrication of C4s (Controlled Collapse Chip Connections, also known as solder bumps or flip chip) for semiconductor substrates. One use for the VEET is to etch metal films which constitute layers of the ball limiting metallurgy (BLM) for the C4s by using the solder as a mask. In the electroetching process as applied to a wafer with the VEET, a metal layer on the wafer is made an anode, the VEET nozzle a cathode, and an electrolyte solution contacts the cathode and the anode to form an electrochemical cell.
FIG. 1 illustrates a nozzle assembly 126 which can be used with a VEET or a HEET (Horizontal ElectroEtching Tool). A plate 128 on the nozzle 126 acts as a cathode and is electrically connected by conventional means to a power supply (not shown). The nozzle includes two hollow portions 140 and 142, and fitting 132 which is connected to a flexible hose 130. Electrolyte solution is pumped from a reservoir (not shown) through the hose and nozzle, and exits the front of the nozzle through holes 129 in plate 128.
The nozzle assembly 126 is fixed to a bracket 112 which moves axially as threaded rod 113 is rotated by drive 114. The nozzle assembly may be moved by any other conventional mechanism as well as by that shown in FIG. 1. The nozzle may be scanned across a workpiece either back and forth or uni-directional generally indicated by arrows 115, and the scan speed is adjustable.
FIG. 2 illustrates the nozzle 126 in operation in a VEET. A wafer 200 is positioned vertically in a fixture 202 and held to face plate 203 by clips 204. Voltage is applied to the wafer (via the face plate and the clips) and to plate 128, and electrolyte solution 206 flows under pressure through the nozzle. A linear jet of etching solution is thus created as the nozzle 126 is scanned in the direction of arrow 208, and gravity causes the solution 206 to flow downward in the direction of arrows 210.
A problem (thoroughly discussed in the copending application) that occurs with vertical electroetching is excessive and uncontrolled undercutting of the layers of metal in the BLM, i.e. that they will be overetched horizontally under the solder which functions as a mask during etching. Undercutting to some extent is desirable, and control of the undercut allows control of the diameter of the BLM underneath the solder. However, with only partial contact between the plate 128 and the electrolyte solution 206, etching is inefficient and more severe etching conditions such as slower nozzle speed and higher voltages are required in order to remove the BLM in between C4s. These severe conditions result in uncontrolled undercutting.
In an attempt to control undercutting, the electrolyte chemistry and the voltage application were optimized. However, the uncontrolled undercut problem persisted, and was exacerbated by stray currents as the etching solution 206 flowed downward from the nozzle 126 and across the wafer 200, causing a streaking effect.
Thus there remains a need for a vertical electroetching tool which provides full contact between the nozzle and the eletrolyte solution and avoids streaking of the etching solution so that controlled undercutting is possible.